Temperature elevation of various Drosophila melanogaster tissues and cultured cells results in rapid and profound changes in several aspects of gene expression. It is the aim of this proposal to utilize the so called "heat shock response" to study mechanisms of translational control of protein synthesis in this eukaryote. When tissue culture cells are shifted from 25 degrees to 25 degrees C there is a rapid decline in the synthesis of pre-existing proteins. Shortly thereafter, the vigorous and sustained synthesis of a small number of characteristic heat shock proteins is initiated. We have recently shown that pre-existing messages are not simply destroyed following temperature elevation, but are in fact competent to function again in translation when the temperature is returned to normal. Thus, at elevated temperatures, one set of messages is discriminated against at the level of translation, while another coexisting set is vigorously translated. The mechanism of this discrimination will be the primary focus of this investigation. Our approach to this problem will first be to analyze the in vitro translation activity of various Drosophila messenger RNA fractions in a heterologous cell free system. If the effects of temperature on message selection can be duplicated in vitro using purified messenger RNAs, some intrinsic property of the message itself, such as its ability to form certain base pairings, would be implicated in this process. If the temperature effects are not duplicated in heterologous cell free extracts, investigations will center upon whethr some reversible modifications are made on pre-existing messages which causes their exclusion from translation, or, alternatively, whether certain Drosophila translation factors are responsible for this discrimination.